The invention concerns a filler metal for welding aluminum alloys, in particular for welding alloys of the AlZnMg type.
AlZnMg alloys have found wide application because of their good weldability, in particular because the weld region hardens at room temperature to the strength level of the parent metal. Initial difficulties, which were due to poor resistance to stress corrosion, were overcome by choosing the appropriate alloy composition, for example, by having an appropriate Zn/Mg ratio and by suitable heat treatment, for example, multi-stage artificial aging.
It is also know that additions of copper in amounts up to 2.0% to the AlZnMg type alloy raises the strength and to a large extent prevents stress corrosion cracking from occurring.
In using these alloys in welded constructions, however, it has been found that the welds meet the requirements regarding stress corrosion and exfoliation corrosion susceptibility only when the construction has been heat treated as a whole. It has been found impossible to comply with this requirement, in particular in the case of large welded constructions.
Various efforts have been made to improve the corrosion resistance of the weld by means of suitable filler metals. Thus, for example, in the Aluminium Taschenbuch, 13th issue, Page 551, non age-hardenable alloys of the type AlSi, AlMg and AlMgMn have been suggested for welding AlZnMg 1. The corrosion problems are indeed solved this way, but only low weld strengths can be achieved with these materials. It is clear, therefore, that the high strength values which can be reached with AlZnMg alloys in welded constructions cannot be exploited with these materials.
Attempts have already been made to use AlZnMg alloys as filler metal. Thus, for example, in the German Patent DT-OS No. 22 34 111, an age-hardenable aluminum filler metal of the following composition has been proposed: zinc from 2.0 to 6.0%, magnesium from 1.5 to 5.0%, chromium from 0.1 to 0.7%, silver from 0.05 to 1.04%, bismuth from 0.001 to 1.0%, beryllium from 0.001 to 1.0%, zirconium from 0.05 to 0.2%, less than 0.4% manganese, less than 0.2% silicon, less than 0.5% iron, and less than 0.08% copper.
The mechanical properties in the weld which can be achieved with this filler metal are comparable with those of the parent metal. This filler metal also allowed the requirements regarding stress corrosion susceptibility to be satisfied to a large degree. It has been found, however, that in spite of optimal heat treatment of the welded construction, there is relatively large susceptibility to weld boundary corrosion. Therefore, although adequate strength values are obtained with such welds, there are risks involved in their use in corrosive surroundings.